As reported in parent application Ser. No. 07/788,151, the utilization of polysulfide in an amount of about 0.5-8% on wood, expressed as elemental sulfur, when used in conjunction with extended modified continuous cooking, produces a kraft pulp with good Kappa, viscosity and yield. It has now been found according to the present invention that the high sulfide content white liquor that is produced according to the parent applications is eminently suited for use in the production of polysulfide so as to get the advantageous results described in parent application Ser. No. 07/788,181.
According to the invention, the hydrogen sulfide containing gas stream generated during the treatment of organic sulfur gases is contacted with white liquor to form a solution containing sodium hydrosulfide. The sodium hydrosulfide containing liquid is then reacted with oxygen or an oxygen containing gas (containing sufficient oxygen to get the desired results) under suitable conditions while in the presence of a suitable catalyst to yield sodium polysulfide and sodium hydroxide. Hydrogen sulfide may also be generated in situ by the addition of sulfur, in the form of elemental sulfur, which may be generated in processing according to the present invention (e.g. in a fuel cell). Thus according to the invention, it is possible to very effectively produce and utilize a sodium polysulfide containing cooking liquor when treating black liquor.
According to one aspect of the present invention, a method of kraft pulping of comminuted cellulosic fibrous material with white liquor containing sodium polysulfide, and creation of the polysulfide in the white liquor from a first gas stream including over 10% by weight organic sulfur compounds, is provided. The method comprises the following steps: (a) Adding hydrogen to the gas in the first gas stream. (b) Passing the first gas stream, in the presence of the added hydrogen, past a hydrogen desulfurization catalyst to produce a second gas stream containing primarily hydrogen sulfide and sulfur free fuel gas such as methane, carbon monoxide, hydrogen, and ethylene. (c) Bringing the second gas stream into operative contact with white liquor and with other chemicals, and under selected conditions, effective to produce sodium polysulfide in the white liquor. And, (d) using the white liquor, with sodium polysulfide, in a kraft process to treat comminuted cellulosic fibrous material to produce cellulosic pulp. Typically steps (c) and (d) are practiced to produce and utilize white liquor containing about 0.5-8% sodium polysulfide on wood.
Alternatively, instead of steps (a) and (b), there may be the step (a1) of effecting substoichiometric combustion of the gas in the first stream to produce the second stream. The actual production of the polysulfide may take place in a number of different manners. For example step (c) may be practiced by utilizing a fuel cell, which ultimately produces electrical energy. Fuel cells which can be used for these purposes is described in U.S. Pat. Nos. 4,544,461 and 4,320,180, the disclosures of which are hereby incorporated by reference herein. According to this procedure, the second gas stream is reacted with the fuel cell with some form of gaseous oxygen to produce elemental sulfur. Then the elemental sulfur is added to the white liquor so as to produce sodium polysulfide in the white liquor, such as according to the formula: EQU 2 NaHS+2x-1 S.fwdarw.2 NaS.sub.x +H.sub.2 S.
As another alternative, step (c) may be practiced by utilizing the MOXY TM process in which a wet-proofed activated carbon catalyst is utilized, such as described in U.S. Pat. No. 4,024,229 (the disclosure of which is hereby incorporated by reference herein). According to this method, a second gas stream is passed into contact with clarified white liquor, and then the clarified white liquor is passed into contact with a wet-proofed activated carbon catalyst and some form of gaseous oxygen to promote the generation of sodium polysulfide in the white liquor. Typical reactions for producing sodium polysulfide according to this procedure are as follows: EQU H.sub.2 S+NaOH.fwdarw.NaHS+H.sub.2 O EQU 2 NaHS+O.sub.2 .fwdarw.2 S+2 NaOH EQU Na.sub.2 S+S.fwdarw.Na.sub.2 S.sub.x
According to another aspect of the invention, step (c) is practiced by passing the second gas stream into contact with unclarified white liquor (having metals present therein), and some form of gaseous oxygen, to produce white liquor containing sodium polysulfide, the reaction catalyzed by lime mud existing in the unclarified white liquor. E.g. see U.S. Pat. No. 5,082,526. Then, the white liquor is clarified to remove the lime mud from it.
The gas in the first gas stream includes substantial amounts of water vapor, and there is also typically the further step (e) of drying the gas in the first gas stream before the practice of step (a). Also there is preferably the further step of utilizing a second white liquor, having a significantly lower sulfur content, and polysulfide (if any) content, than the white liquor produced in step (c) and utilized in step (d), to treat the comminuted cellulosic material after step (d).
According to another aspect of the present invention, a method of kraft pulping of comminuted cellulosic fibrous material with white liquor containing sodium polysulfide, and creation of the polysulfide in the white liquor from off gases from black liquor treatment, is provided. This method comprises the following steps: (a) Acting upon black liquor to obtain off gases containing organic sulfur compounds, and collecting the off gases. (b) Treating the off gases to produce a gas stream containing primarily hydrogen sulfide and methane or other non-sulfur containing fuel gases. (c) Bringing the gas stream into operative contact with white liquor and with other chemicals, and under such conditions, effective to produce sodium polysulfide in the white liquor. And, (d) using the white liquor, with sodium polysulfide, in a kraft process to treat comminuted cellulosic fibrous material to produce cellulosic pulp. Step (c) in each case may be practiced in the same manner as step (c) according to the first aspect of the invention, that is by utilizing a fuel cell, the MOXY TM process, or an oxidation reaction with unclarified white liquor.
According to yet another aspect of the invention, an apparatus for producing white liquor having sodium polysulfide therein is provided. The apparatus comprises the following elements: Means for acting upon black liquor to produce organic sulfur containing off gases, and collecting the off gases (such as shown in U.S. Pat. No. 4,929,307, the disclosure of which is hereby incorporated by reference herein). Hydrogen desulfurization means for reacting the black liquor off gases (e.g. with hydrogen in the presence of a catalyst, or substoichiometrically) to produce primarily hydrogen sulfide and methane. A hydrogen sulfide fuel cell system for producing electrical energy. A conduit connecting the hydrogen desulfurization means to the fuel cell. A white liquor inlet to the fuel cell, a fuel gas outlet from the fuel cell, an oxygen containing gas inlet to the fuel cell, and a polysulfide containing white liquor outlet from the fuel cell. Means for treating comminuted cellulosic material with polysulfide containing white liquor. And, a conduit connecting the fuel cell polysulfide containing white liquor outlet to the means for treating comminuted cellulosic material with polysulfide containing white liquor.
It is the primary object of the present invention to provide a simple and effective way of producing sodium polysulfide in white liquor for the enhanced kraft cooking of pulp. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.